Quenching baffles for an electrical overload fuse

ABSTRACT

An overload fuse with a fusible conductor and quenching baffles is curved preferably forming a partial annulus, and several quenching baffles are disposed perpendicular to the fusible conductor. Due to its flat design, this fuse has only a small volume and at the same time a large switching capacity because the partial arcs formed between the quenching baffles are driven radially outward by the dynamic forces and are lengthened accordingly.

BACKGROUND OF THE INVENTION

This invention relates to electrical overload fuses in general and moreparticularly to an electrical overload fuse with a fusible link andquenching baffles of electrically conductive material.

As is well known, a fuse has as its purpose disconnecting part of anetwork or an electrical load in case of an overload or a short circuit.The fusible conductor is designed so that it melts as soon as theconductor current reaches a predetermined limit. By the specialmechanical design of the fuse, the arc is forced to assume an operatingvoltage which is higher than the driving line voltage.

In the known fuses, the fusible conductor has essentially two purposes,namely, the current carrying function for the current flowing duringnormal operation and, for an overcurrent, which must likewise be carriedfor a certain period of time in the case of a disturbance or shortcircuit. It also has the current interrupting function which is obtainedby a sufficiently large countervoltage. The two functions, however,place contradictory requirements on the fusible link, which, for highervoltages, leads to relatively complicated shapes of the fusibleconductor.

A sufficiently high arc voltage can be produced by the fusible link,after it has melted through, only with a correspondingly great length.This, however, means a correspondingly large voltage and power dropduring normal operation. In order to reduce this voltage drop, fusiblelinks with several constrictions have been used. This design, however,raises a further problem, namely, the simultaneous melting of allconstrictions (U.S. Pat. No. 1,946,553).

But even with this known design the voltage drop is still several tenthsof a volt. In systems with a larger number of such fuses, for instance,in static converters with thyristors, with each of which a fuse isassociated, a considerable amount of dissipation loss is thereforegenerated which must be removed as heat.

The problem of designing the fuse so that its voltage drop is smallduring normal operation and so that it is, at the same time, able tobuild up a large countervoltage which acts as the quenching voltage forinterrupting the current, therefore arises.

In one known embodiment of a fuse, quenching baffles of electricallyconductive material are therefore provided having flat sides extendingtransversely to the longitudinal direction of the fusible conductor andarranged one behind the other in the longitudinal direction thereof. Asthe fusible conductor melts, partial arcs are produced between thequenching baffles. Alternating with the quenching baffles are spaces inthe form of hollow cylinders of a material which gives off a gas whichacts as a quenching gas under the action of the arc. The fuse musttherefore be pressure proof. The sum total of the individual partialarcs acts as a quenching voltage. The quenching baffles extendperpendicular to the direction of the fusible conductor and are arrangedone behind the other in the direction of the fusible conductor. Forhigher switching voltages, especially above 1000 volts and with acorrespondingly large number of quenching baffles, a relatively largedesign of the fuse is obtained.

SUMMARY OF THE INVENTION

According to the present invention, the abovementioned problem now issolved by using a curved conductor and disposing several quenchingbaffles at least approximately perpendicular to the fusible conductor.Through the quenching baffles which are arranged perpendicular to thecurved fusible conductor, in particular radially to a circular fusibleconductor, a flat design of the fuse is obtained, the height of which isnot substantially greater than the height of the quenching baffles. Themutual spacing of the quenching baffles increases in the directionradially outward. The partial arcs generated after the fusible conductorhas melted, are therefore driven outward due to the electrodynamicforces. The lengths of the individual partial arcs and the total lengthof the arc are increased, and the switching voltage is increasedaccordingly. Because of these electrodynamic forces, the fusibleconductor, in a heated condition, is at the same time pressed againstthe baffle, and increased cooling is ensured thereby.

One embodiment of the fuse for higher voltage with a large number ofquenching baffles has a fusible conductor which forms a helix, havingtwo or more turns are arranged one over the other.

In one embodiment of the fuse, the quenching baffles can rest directlyagainst the fusible conductor. The fusible conductor is cooled by thebaffles and can carry a current which is substantially larger than thecurrent which is obtained from the cross section of the uncooled fusibleconductor. The fusible conductor can advantageously be arranged on thecylindrical surface of a hollow cylindrical core. The ends of thequenching baffle facing away from the fusible conductor can preferablybe arranged in slots in the inner wall of a hollow cylindrical housingof insulating material. In that case the dimensional tolerances of thequenching baffles can be correspondingly larger.

In some circumstances it may be advisable to design the quenchingbaffles in such a manner and to arrange them around the fusibleconductor so that their ends are opposite the fusible conductor with apredetermined spacing. The cooling effect of the quenching baffles thensets in only at a predetermined current.

In a special embodiment of the overload fuse, the ends of the quenchingbaffle facing the fusible conductor may be provided with a coating oflow electric conductivity which may, however, have high thermalconductivity. Also the fusible conductor may be provided with such acoating instead of the quenching baffles.

Parts of the hollow cylindrical housing, preferably the outer tube, mayconsist of gas permeable material, especially filter ceramics so thatoverpressure that may occur can be relieved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross section an embodiment of a fuse according tothe present invention.

FIGS. 2 and 3 each show part of a special embodiment of the fuse.

FIGS. 4 and 5 each show part of a special design of the quenchingbaffles.

FIG. 6 illustrates a special design of the fusible conductor.

FIG. 7 illustrates a fuse in a partial annulus with radially extendingbaffles.

FIG. 8 shows the fuse in a helical form.

FIG. 9 shows an intermediate layer between the fuse and baffles.

DETAILED DESCRIPTION

In the embodiment according to FIG. 1, a fusible conductor 2 in the formof an annulus or hollow cylinder is shown. At both of its ends it isprovided with a contact terminal, which are designated as 3 and 4 in thefigure. Extending radially to the fusible conductor 2 are quenchingbaffles 6 of which only a few are shown in the figure, and the positionof the others is merely indicted by dashed lines. The fusible conductoris arranged on the outer surface of a core 8 of insulating materialwhich preferably has the shape of a hollow cylinder. This assembly isarranged in a housing 10 which may preferably consist of insulatingmaterial, particularly ceramics.

According to FIG. 2, which is a cross-section along line II--II of FIG.1, one end of the quenching baffles 6 rest against the fusible conductor2 which has the form of a cylinder. The outer ends of the quenchingbaffles extend into slots 12 in the housing 10. Similarly, the lower andupper end faces of the quenching baffles 6 can be arranged in slots in abase plate 14 and a cover plate 16, respectively.

The housing 10 and, optionally, also at least the outer part of the baseplate 14 and the cover plate 16 may advantageously consist of a gaspermeable material, especially a so-called filter ceramic. The arc canbe prevented from restriking at the outer surfaces of the fuse by makingthe holes not substantially larger than 1 mm, and, in particular,smaller than 1 mm.

In the embodiment according to FIG. 3 which shows part of a crosssection similar to the section of FIG. 1, the fusible conductor 2 isarranged between the core 8 and the housing 10 in such a manner so thatthe quenching baffles 6 extend radially outward as well as also radiallyinward from fusible conductor 2.

If the fusible conductor 2 is connected to the quenching baffles 6 in aform locking manner, all parts of the fuse are heated slowly in the caseof an overcurrent, and after a predetermined time, the fusible conductor2 melts between the quenching baffles 6. The partial arcs drawn betweenthe individual quenching baffles are driven radially outward due to theelectrodynamic forces; the arc length increases with increasing distanceof the quenching baffles 6 and the switching voltage is increasedcorrespondingly. The same electrodynamic forces press the fusibleconductor 2, in the heated condition, against the quenching baffles 6and correspondingly increased cooling is ensured.

In the embodiment according to FIG. 3 with quenching baffles 6 extendingradially outward as well as radially inward, the heat removal from thespaces between the quenching baffles 6 is facilitated. In order toprevent parallel discharge channels, the quenching baffles can, in thisembodiment, also be held in slots in the housing 10 as well as in slotsin the outer cylindrical surface of the core 8. Because of the meandershaped extension of the leakage current paths, increased dielectricstrength is then also obtained.

In the embodiment according to FIG. 4, the quenching baffles 6 aredesigned and arranged about the fusible conductor 2 in such a mannerthat between them and the fusible conductor 2 a gap 18 is produced. Thesize of the gap is chosen so that the arc generated after the fusibleconductor 2 has melted at one point, allows the fusible conductor 2 tomelt further, and its size will generally not be much less than 1 mm. Insome circumstances the ends of the quenching baffles 6 facing thefusible conductor 2 may be provided with a coating 20 which consists ofa material of low electric conductivity, as is indicated in FIG. 5.During the melting and continued melting of the fusible conductor 2,this coating 20 prevents a fusion with one or more quenching baffles 6.The coating 20 may consist, for instance, of a temperature resistantplastic, or a vitreous or enamel-like material.

In the embodiment according to FIG. 6, the fusible conductor 2 isprovided at least partially with a coating 22 which similarly preventsthe mentioned fusion. If a flat, ribbon-like fusible conductor 2 isused, it can be provided with such a coating on its flat side facing thequenching baffle 6. It may be sufficient in some circumstances if,between the fusible conductor 2 and the quenching baffles, anintermediate layer provided with openings is arranged, the openings ofwhich allow the arc to pass after the fusible conductor 2 is melted.

FIG. 7 shows a fusible conductor 2 arranged in a partial annulus andhaving quenching baffles 6 extending radially outward.

In FIG. 8, the construction is similar to that of FIG. 7 except thatconductor 2 takes a helical form with a plurality of turns, one over theother.

FIG. 9 shows a view similar to that of FIG. 2 where an intermediatelayer of a material of low electrical conductivity 24 is disposedbetween the baffles 6 and conductor 2. Gaps 26 are formed to providepassage openings for an arc.

What is claimed is:
 1. In an electrical overload fuse with a fusibleconductor and quenching baffles of electrically conductive material, theimprovement comprising, the fusible conductor being curved along aradius of curvature about an axis, forming a continuous curve from oneend to the other and a plurality of quenching baffles disposed at leastapproximately perpendicular to the fusible conductor and extendingradially with respect to said axis.
 2. The improvement according toclaim 1, wherein said fusible conductor is disposed on the outer surfaceof a hollow cylindrical core.
 3. The improvement according to claim 2,wherein said quenching baffles contact said fusible conductor.
 4. Theimprovement according to claim 2, wherein the ends of said quenchingbaffles are spaced from the fusible conductor.
 5. The improvementaccording to claim 4 wherein the ends of said quenching baffles have acoating of a material of low electric conductivity.
 6. The improvementaccording to claim 4, wherein said fusible conductor is at leastpartially covered with a coating of a material of low electricconductivity.
 7. The improvement according to claim 4, and furtherincluding an intermediate layer of a material of low electricconductivity having passage openings for the arc disposed between thefusible conductor and the quenching baffles.
 8. The improvementaccording to claim 2 and further including a hollow cylindrical housingof insulating material having slots on its inside wall surrounding saidconductor and baffles, said quenching baffles extending into said slots.9. The improvement according to claim 7 at least part of said housingconsists of a gas permeable material.
 10. The improvement according toclaim 9, wherein said gas permeable material is filter ceramic.
 11. Inan electrical overload fuse with a curved fusible conductor andquenching baffles of electrically conductive material, the improvementcomprising the fusible conductor extending in a partial annulus and aplurality of quenching baffles extending radially from the fusibleconductor over said annulus.
 12. In an electrical overload fuse with acurved fusible conductor and quenching baffles of electricallyconductive material, the improvement comprising the fusible conductorextending in a helix and said quenching baffles extendingperpendicularly from said fusible conductor over the helix.